Prostate cancer is the most common malignant cancer in North American men. It is estimated that approximately 200,000 new cases and 31,500 prostate cancer-related deaths will occur in the United States annually. Prostate cancer is now the second leading cause of cancer death in men, exceeded only by lung cancer. It accounts for 29% of all male cancers and 11% of male cancer-related deaths.
Currently, the FDA has approved serum PSA (prostate-specific antigen) for use as a prostate cancer screening laboratory test. Like many serum tumor markers, PSA is produced by both normal and cancerous glands. In men with prostate cancer, the serum levels can be elevated with both localized and advanced or disseminated disease. PSA levels are generally proportional to the volume of the cancer. Because there is a significant overlap between PSA levels found in cancer and benign prostatic hyperplasia, it is important to obtain sequential levels in low or borderline elevated values.
The introduction of free PSA (fPSA) testing has introduced a greater level of specificity in identifying early prostate cancer. In 1998, the FDA approved fPSA testing as a diagnostic aid for men with total PSA values between 4.0-10.0 ng/mL. This has often been the diagnostic gray zone for total PSA testing and fPSA may aid in the stratification. In general, at any free PSA level, the more enlarged the prostate, the more likely the prostate may be cancerous. However, these tests remain qualitative at best, and more reliable types of detection, and means for staging the cancer treatment, are needed.
Prostate cancer, like other forms of cancer, is caused by genetic aberrations, i.e., mutations. In the mutant cells the normal balance between the factors that promote and restrain growth is disrupted, and as a result, these mutant cells proliferate continuously—the hallmark of tumor cells. Mutations can arise spontaneously or by external factors such as chemical mutagens, radiation, or viral integration, which inserts extra-genomic DNA that may or may not contain an oncogene. A cellular gene can be modified by point mutation, insertion and frame shift (including truncation), (functional) deletion (including silencing), or translocation, which sometimes can result in gene fusion. In this way protooncogenes can become oncogenes, which promote proliferation, and tumor suppressor genes can become inactivated, also inducing tumor growth. Any combination of the above-mentioned changes in DNA can contribute to tumor formation. The consequences of these changes may or may not be held in check by the immune system (immune surveillance).
One protein whose expression has been implicated in certain cancers is the Glypican 3 protein, or GPC3, a heparin sulfate proteoglycan anchored to the cell membrane via glycosylphosphatidylinositol (1). The protein has a molecular weight of 65.6 kDa and the polypeptide chain has 580 amino acid residues. The heparin sulfate chain of the proteoglycans interacts with heparin-binding growth factors and thus serves as a co-receptor in cell signaling (2), although GPC3 might bind also in a different way (3). In embryonic development, GPC3 modulates BMP and EGF-mediated effects during renal branching morphogenesis (4). It also controls cellular responses to BMP4 in limb patterning and skeletal development (5). Except for weak expression in bronchiolar epithelial cells, GPC3 is not expressed in systemic organs (6). Because its expression is decreased in lung adenocarcinoma (7, 8), human gastric cancer (9), ovarian cancer cell lines (10, 11), mesotheliomas (10, 11), and breast tumors (12), GPC3 may function as a tumor suppressor. But because its mRNA and protein expression is increased in hepatocellular carcinomas (13-15), colorectal malignancies (16), and embryonal tumors (17) as compared to normal tissue, GPC3 is also considered an onco(fetal) protein. For hepatocarcinoma, GPC3 is a promising diagnostic marker (6). It is also known that in these cancers, GPC3 modulates FGF2 and BMP-7 signaling (18), and promotes growth by stimulating canonical Wnt signaling (3).
In more than 80% of melanoma and melanocytic nevus, both GPC3 mRNA and protein are expressed (19). Interestingly, GPC3 protein is found in sera from 40% of melanoma patients but not in sera from subjects with large congenital melanocytic nevus and from healthy donors. GPC3 expression disappeared in sera from one third of the patients after surgical removal of the melanoma (19).
Heretofore, there has been no demonstrated link between changes in GPC3 levels and prostate cancer. Such a link could have a number of important diagnostic and therapeutic applications. In accordance with the present invention, it has now been discovered that (i) GPC3 levels increase significantly in prostate cancer cells, and (ii) this increase can be measured the ductile fluid and blood-fluid sample of patients.